Important Astronomers, their Instruments and Discoveries

by Paul M. Rybski

Part 3

The Story of the Earliest Reflecting Telescopes

Proposals for Metal-mirrored Reflecting Telescopes

Italian monk Niccolo Zucchi proposed (1616) a concave mirror to replace the refractor's objective lens. The poor shape of his mirror, added to the lack of means of viewing the image without blocking the mirror, caused Zucchi to give up on the idea.

French astronomer Marin Mersenne proposed (1630) developing a reflector that used two concave mirrors, the smaller secondary mirror to reflect the light through a hole in the center of the larger primary mirror. Unfortunately, he was dissuaded from working on it further by French mathematician RenŽ Descartes.

Scottish mathematician James Gregory proposed (1663) a two-mirror reflector with a concave, paraboloidal primary and concave, ellipsoidal secondary, the secondary bringing light to a focus inside the telescope tube and a convex eyepiece delivering light to the eye. Sadly, he did not construct a working version because he could not find a craftsperson skilled enough to grind and polish the required mirrors.

Early Work on Metal-mirrored Reflectors

English mathematician and physicist Isaac Newton (1648-1727) designed and constructed a two-mirror reflector (1668), a concave primary delivering a converging cone of light to a flat secondary mounted at a 45ˇ angle to the optical axis which directed the light to a focus outside the tube where an eyepiece could be placed. A second such reflector was built and presented to the Royal Society in January of 1672.

Frenchman N. (or G.) Cassegrain proposed a two-mirror reflector (1672) with a concave, parabolic primary mirror and a convex, hyperbolic secondary, the secondary directing a converging cone of light out the centrally perforated primary like the Gregorian and to a focus outside the telescope tube. Subsequent analysis revealed that surface errors in the Cassegrain secondary can be made to cancel those in its primary, while this cannot be done in a Gregorian. Newton analyzed this system incorrectly, and his criticisms caused Cassegrain to withdraw the idea and sink into obscurity.

British astronomer John Hadley (1682-17XX?) produced the first reflector (1721) equal in performance to the 123 foot refractor f 7.5 inch aperture donated to the Royal Society by Christian Huygens in 1692. Mirrors for this and a few later reflectors by Hadley were manufactured from metal alloys prescribed by British astronomer Samuel Molyneux. Hadley is remembered more for construction of the first navigation sighting device (the octant) which permitted position estimates accurate to one nautical mile, a device upon which the modern marine sextant is based.

Scottish minister and optician James Short (c. 1708-1768) constructed the highest quality reflectors to that time, most of Gregorian design, from 1734 until his death in 1768. While he regarded as proprietary his techniques for parabolizing primary mirrors, his optical workmanship served as a standard to match or exceed in instruments constructed by others after his death. After Short's death, English physician John Mudge (17XX-17XX) and English minister John Edwards (17XX-17XX) perfected metal alloys for mirrors and created and published information on casting and polishing them, information Short must have possessed but which he destroyed before his death.

The Best Metal-mirrored Reflectors

William Herschel (1738-1822), who became the greatest observational astronomer of the late eighteenth and early nineteenth centuries, was born in Hanover, then an independent port city, to a father who taught his sons to be musicians in the band of the Regimental Guard. Loss of Hanover to the French in 1757 propelled William and his brother Alexander to England, where in 1766 William became organist in the Octagon Chapel in Bath. His fortune made with plentiful concerts given and students instructed, William was able to devote some of his energy to his hobby of astronomy and could bring his sister Caroline to England in 1772.

Herschel constructed four refractors of increasing aperture and length in 1773. Using a rented Gregorian, Herschel showed this telescope superior to the refractors in ease of use, so William tried unsuccessfully to make one himself; The results being poor, he instead made a Newtonian that worked very well on Saturn and the Orion Nebula. By 1774, two more Newtonians were constructed and in use, the first of 4.3 inch aperture and 6.5 foot focal length, the second of 9 inch aperture 10 foot focal length.

Because his new telescopes produced images ten times smaller than those delivered by telescopes used to catalog star positions, Herschel began a survey of all cataloged stars to identify those which were truly single whose positions could be trusted from those which were multiple whose positions would have to be remeasured. Also he sought close pairs of extreme brightness difference to find evidence of stellar parallax that would yield a distance to stars exhibiting the effect.

On March 13, 1781, he discovered what he thought was a new comet but which, on calculation of its orbit, proved to be a planet outside Saturn's orbit. This discovery got the attention of professional astronomers whose telescopes proved inferior when compared directly with Herschel's. This discovery also got the attention of the King who made him "Royal astronomer", freeing Herschel from ever again having to make his living in music.

Shortly thereafter, Herschel completed a 12 inch aperture telescope of 20 foot focal length, followed by a 19 inch aperture telescope also of 20 foot focal length. These he used to continue his double star research as well as to catalog all visible nebulae -- the latter triggered by the publication in Paris of French astronomer Charles Messier's catalog of 103 nebulous objects -- and begin a star counting project which would yield a picture of the structure of the stellar universe around the Sun.

During his double star survey, he discovered "binary stars", stars that formed at the same time and were close enough to be orbitting one another. He extended our knowledge of the solar system by defining the direction of rotation of Mars and refuting claims of mountains on Venus. Herschel added to his meagre income as the King's astronomer by constructing reflecting telescopes for wealthy patrons. In 1786, Herschel began the construction of his largest telescope, one of 48-inch aperture and 40 foot focal length. On 27 August 1789, it was completed. On its second night's use, Herschel discovered the sixth satellite of Saturn, now named Enceladus.

While the 40 foot remained the largest telescope in the world, Herschel completed most of his remaining research with the 20 foot and with a 24 inch aperture reflector of 10 foot focal length. The work completed by Herschel with these instruments was unprecedented in scope, volume and quantity: a second catalog of nebulae in 1789, a third in 1802; the seventh satellite of Saturn located as well as the determination of Saturn's rotation period; measuring the true brightness of stars, including the study of stars that vary their brightness; the first observation of spectra of stars beyond the Sun; the discovery that nebulae consisted of at least two classes, one consisting of stars and that appeared gaseous; study of the comets of 1807 and 1811; the discovery of infrared radiation; and finding the Sun was moving among the stars in the direction of the constellation Hercules.

His sister Caroline played an essential part in most of Herschel's telescopic discoveries, writing down his observations as they were made and freeing him to remain at the eyepiece; Caroline made important observations in her own right, having discovered six comets in her own work and having corrected many errors in Flamsteed's star catalog when they were discovered to be discrepant with Herschel's own.

Herschel contributed to the discovery of minor planets ("asteroids" as he called them) by constructing a telescope used and copied by Johann Schršter, a minor official in the Hanoverian suburb of Lilienthal who started the Societas Lilientalica with the expressed purpose of discovering the "missing" planet predicted by Bode's Law between the orbits of Mars and Jupiter. Between 1801 and 1807, the minor planets Ceres, Pallas, Juno and Vesta were discovered by members of this society.

Herschel appears to have been the first to investigate the resolving power of the human eye and that of telescopes. Given limited instrumentation, he found that telescope resolution was inversely proportional to aperture diameter; he did not determine resolution's dependence on the wavelength of the observation.

For those who would like to investigate the history of modern refracting and reflecting telescopes, I include the following names and dates:

  1. Other Astronomers and Instrument Builders Important in the History of the Telescope.
  2. Achromatic Refracting Telescopes.
  3. Glass-mirrored Reflecting Telescopes.

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Bill Arnett); last updated: 1997 Jan 12